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Actually the warning here is correct. The parentheses around _cost do nothing to associate it with sqrt - this would only be guaranteed if you wrote

(sqrt _cost)/(1+_accuracy)

Alternatively you may have intended

sqrt (_cost/(1+_accuracy))

In practice most arithmetic operators are evaluated from left to right so the code as written would have been evaluated as the first option but the warning is intended to highlight that the statement is potentially ambiguous.

The reason for the warning message is that associativity depends quite a lot on the operators being used (there are some underlying rules but they are not necessarily obvious to your average coder). For example...